When Chemists Attack

A brass is an alloy (or solution) of copper and zinc metals. Brasses have been known for a long time, and have a pleasing color – somewhat like metallic gold.

Regular pennies are made of copper metal – sometimes. Pennies made in or after 1983 aren’t entirely copper, since copper got too valuable to throw away by just making pennies with it. New pennies are actually made of zinc, with a thin copper coating (so they still look like copper pennies). This gives the pennies some rather neat chemical properties, which I’m going to post about in a few posts here on the blog.

If new pennies are made of zinc with a thin copper coating and brasses are mixtures of copper and zinc, why can’t we turn a penny into brass and give it a cool-looking "gold" finish? Well … we can. It’s quite easy to do with a new penny. (It’s even possible with an old copper penny – but you have to add the zinc yourself.)

Since a brass is a mixture of copper and zinc, we have to get the atoms of copper to mingle with the atoms of zinc. To do that, we need to get them moving. While atoms are essentially always moving, in the solid state they don’t move very much – so simply having the copper in contact with zinc is not enough to make a brass – at least not in a reasonable timeframe. (If it were that easy, then your pennies would be brass already!) We need to stir things up a bit at the atomic level, and that means we need to apply heat.

We could apply enough heat to simply melt the zinc and copper and let them mix to form a brass, but that wouldn’t be very fun. After all, we want a brass penny – not a lump of brass. So we don’t melt the penny – we heat it up gently so that the brass forms without melting the penny.

To get a nice shiny brass penny, you need to start with a nice shiny (post-1983) copper penny. Clean the penny with some steel wool, available at just about any store that sells household supplies. Buff the penny with the steel wool until it’s bright and shiny. Try to avoid buffing too hard, since you don’t want to remove the copper or scratch the penny badly.

Even if the penny is new and appears clean, buff it a little. You’re more likely to get a nice, evenly colored brassy penny that way.

Now, you’ll need a heat source, preferably one that can reach at least 200^oC – same thing as 392^oF. I used a hotplate with adjustable temperature settings for the pictures in this post, but you can also use a toaster oven. (A stovetop or grill could work, but it would probably be too difficult to control the temperature. Don’t even think about attempting this in a microwave oven.)

If you set your heat source to about 400^oF, it will take approximately 25 to 30 minutes for your penny to become brassy. A temperature of 250^oC (482^oF) makes the process takes less time, but seems to produce poorer-looking results. If you’re using a small toaster oven like the one I have at my house, you might have to bump the setting to 425^oF to get good results. You may also have to wait a little longer than 30 minutes. You will probably have to experiment a bit with temperature and time, but at least the raw materials are cheap!

Want to see what will happen? The pictures below were taken at the 250^oC temperature. The penny goes through the same color transformations at lower temperature – only more slowly.

At two minutes, you can see the penny beginning to change color and darken.

Two minutes at 250^oC

Another minute in, the penny takes on a silvery tint.

Three minutes at 250^oC

After five minutes, the penny gets a distinctive brass color.

Five minutes at 250^oC

The finished product, removed from the hotplate and cooled after about six minutes of heating. Be very careful with hot metal! It can give you a nasty burn!

Brass penny

Instead of a simple copper coating, the penny is now sheathed in brass. It looks a little like gold, but trying to pass off heat-treated pennies as rare gold coins is not advised!

This entry was posted on Wednesday, June 21st, 2006 at 11:30 am and is filed under Science. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.